This invention relates to lubricants for polymers; more particularly this invention relates to lubricants for polymers made from polymer salts of low molecular weight, copolymers of alpha olefins and alpha, beta-ethylenically unsaturated carboxylic acid.
Lubricants are added to polymers to decrease melt sticking of the polymer, to improve the flow properties, and make it easier to process the polymers. Lubricants generally work to reduce the melt viscosity of the polymer at the processing temperature and/or to reduce the friction between the polymer and metal surfaces of processing machinery.
The state-of-the-art of polymer lubricants is reviewed in Modern Plastics Encyclopedia, 1979-1980, pages 198-202, at page 675.
Lubricants for polymers which are known in the art include: paraffin wax; polyethylene waxes; calcium stearate; stearate esters, alcohols, and acids; montanbased esters, acids, and salts; stearate salts; and amide waxes.
Lubricants can act internally or externally depending on their compatibility with the polymer. Generally the lubricant has a lower melting temperature than the polymer which it is lubricating. An external acting lubricant does not blend well with the polymer and maintains its separate integrity. The external lubricant melts and reduces polymer to metal friction which can cause stickiness between the process machine and polymer.
Internal and external lubrication characteristics are influenced by the degree of compatibility between the lubricant and the polymer. Compatibility by definition is the ability of two or more constituents to mix and remain homogeneously dispersed in one another. Physical compatibility depends on process conditions and physical properties of the lubricant and polymer. Physical properties considered are hardness and viscosity. Chemical compatibility is based upon chemical structure and interaction of the constituents such as the degree of solubility between the melted lubricant and polymer melt.
The more compatibility between a lubricant and the polymer, whether it is chemical or physical, the more internally the lubricant functions. An internal acting lubricant does not only act at the surface of the polymer, lubricating as it is processed. The lubrication occurs inside the polymer melt as well. The lubricant, if an efficient solvent, forms a continuous solvating layer of molecules around the chain segments of the polymer, decreasing the amount of chain-to-chain contact as well as the chain-to-metal contact on the surface. The internal lubricant intermingles with the polymer melt and forms an intimate matrix. An internal acting lubricant blends into the body of the polymer and affects the flow properties of the polymer. The internal lubricant reduces the polymer-to-polymer friction which would reduce power consumption necessary during processing.
The external function of the lubricant is determined by the incompatibility between the lubricant and the molecule. The more incompatible the lubricant and the polymer are, the more the lubricant acts at the surface of the polymer melt easing the polymer's way through the machinery, such as an extruder. The external lubricant decreases the friction between a metal surface and the polymer.
The amount of lubricant external to the polymer is critical when the polymer is processed. Too much lubricant causes slippage, eliminating the friction necessary for the movement of polymer through the barrel of an extruder. This results in a decrease in output and torque. Polymers are more sensitive to external over-lubrication than to internal over-lubrication. Therefore, the concentration of external lubricants is generally much lower than that of internal lubricants.
It is known in the art to use ionic copolymer additives in various polymers to improve properties. The ionic copolymers disclosed in the art are made from copolymers of alpha-olefins and alpha,beta-ethylenically unsaturated carboxylic acid. However, these copolymers are generally of higher molecular weights and are used to improve polymer properties unrelated to processing, such as impact resistance. Examples of these are U.S. Pat. No. 3,264,272 and U.S. Pat. Nos. 3,404,134, 3,347,957 and 4,210,579. The high molecular weight of the copolymer acids used to make the ionic copolymers are reflected by the fact that these higher molecular weight copolymers have measurable melt indexes, and that they can be processed by milling.